The geometry of the inlet structure and more particularly the shape of the cowling which defines the inlet passageway into an aircraft gas turbine engine has important effects on engine performance. As flight conditions for the aircraft vary, the character of the airflow approaching and entering the engine will change. To accommodate this variation, it is beneficial to vary the geometry of the inlet and the shape of the cowling. More particularly, at cruising speeds a cowling having a sharp aerodynamic surface helps to minimize drag and promote efficient airflow in and around the engine. This is especially true at high subsonic cruising speeds above mach 0.85 at which a contraction ratio (ratio between the area of the highlight cross section and the area at the throat of the inlet) of approximately 1.1 is necessary for performance. However, at slower speeds and higher angles of attack, the cowling defining the intake passageway should present a more blunt aerodynamic contour to the incoming airflow in order to prevent distortion of the airflow resulting in the formation of eddies when the air passes by the cowling structure as can seriously degrade engine performance. This tends to be true during takeoff and landing as high angles of attack are normal during these maneuvers and therefore a contraction ratio of approximately 1.25 or greater is considered necessary.
In the past, these problems have been addressed through the use of structures which define variable inlet geometries for aircraft gas turbine engines by changing the aerodynamic characteristics of the cowling surrounding the intake passageway in accordance with flight conditions. These structures have commonly comprised annular lips mounted on rails that may be translated forwardly from the cowling structure to allow a more blunt aerodynamic contour to be presented to the oncoming air. Such structures have been relatively successful in avoiding airflow distortion problems. However, they have, unfortunately, also been the source of substantial noise generated as a result of the interaction between the airflow and the support rails mounting the lip. This noise is generated during takeoff and landing when the lip is deployed in its forward position, and at a time when extra noise is least desirable since it may tend to disturb the public in and around the airport which the aircraft is approaching or leaving.
It is, therefore, an object of the present invention to provide an improved system for efficiently varying the inlet geometry of the cowling defining the intake passageway of an aircraft gas turbine engine so that the aerodynamic contours of the cowling can be changed in accordance with flight conditions.
It is another object of the present invention to provide an improved structure adapted for changing the aerodynamic contour of the cowling defining the intake passageway into a gas turbine engine of an aircraft, which structure is constructed and arranged to reduce the amount of noise generated by the interaction of the structure with the airflow into and around the engine.
It is a further object of the present invention to provide an improved system for supporting operational structures for use in and around the intake passageway into an aircraft gas turbine engine, which system is adapted for noise reduction.